1. Field of the Invention
This invention relates generally to electromagnetic signal receiving devices, and more particularly to an electromagnetic signal receiving device having a dual elliptical corrugated feed horn.
2. Description of the Prior Art
Direct Broadcast Satellite (DBS) is a point-to-multipoint system in which individual households equipped with a small receiving antenna and tuner device receive broadcasts directly from a geostationary satellite. The satellite receives digital audio and video transmissions from ground stations and relays them directly to individuals. The receiving antenna is comprised of a parabolic reflector designed to collect the satellite signals and focus them at the focal point, where a Low Noise Block with integrated Feed (LNBF) module is mounted to convert the incoming signals to a lower frequency band and transmit it to a tuner device. Microwave signals aligned to the axis of the parabolic reflector are collected at the focal point, where the LNBF module is located. The LNBF module also acts as a filter and an amplifier to selectively boost the signal received by the dish collector. The LNBF module comprises a feed for receiving microwave signals and circuitry for processing the received microwaves.
Because of the high sensitivity of these devices and relatively high satellite transmitting power, the parabolic reflector currently being used can be as small as 0.4 meter in diameter. The dishes are mounted outside the home and are manually aligned with the help of a diagnostic display showing received signal strength. Inside the home, a phase-lock loop tuner demodulates the signal from the LNBF module into video and audio signals suitable for a television or stereo tuner.
Normally, each satellite dish antenna is aligned to receive signals from a particular cluster (or group) of satellites in a certain direction. To a dish antenna on earth, the satellites belonging to the same cluster are located so close together that their signals are often indistinguishable from signals radiating from a single satellite. To overcome this problem, when receiving signals from different satellite clusters, more than one dish antenna may be used to point to the different satellites. Another method is to use an electric motor to turn the antenna assembly to point to different satellites. However, employing these methods would make the antenna too complicated and expensive for general home use.
When two satellites (or two clusters of satellites) are separated by a small angle (the angle being larger than the separation angle between satellites within the same cluster), it is possible to use dual LNBF modules placed side by side near the focal point to receive signals from the two satellites. The prior art includes Sharp Corporation's implementation of dual circular corrugated feed horn technology for receiving signals from two satellites shown in FIG. 1. Since the parabolic reflector only has one focus point, the dual circular feed horn needs to be configured to offset the deviation because each of the dual circular feed horn is not in focus. Such a configuration will result in a wider radiation pattern beamwidth, both in the horizontal and vertical directions, than the case of separately aiming at the two satellites by placing the feeds at the focal point of the dish antenna. The wider radiation pattern beamwidth in the horizontal direction can be offset by adjusting the separation of the dual circular feed horn. However, the wider radiation pattern beamwidth in the vertical direction causes an increase in the spill-over loss in the vertical direction. The spill-over loss will show up as a decrease in carrier intensity (of the signal received from the satellite) to noise (C/N) ratio, which will affect signal reception quality. Further, a wider radiation pattern will cause an insufficient rejection for preventing interference coming from the other satellite.
The radiation field pattern of a feed horn is correlated with the width of the horn aperture. The wider the horn aperture is, the narrower the radiation field pattern will become. Though spill-over will be reduced by using a wider horn aperture to narrow down the radiation field pattern of the dual circular feed horn in the vertical direction, this simultaneously narrows down the radiation field pattern of the feed in the horizontal direction. A narrower radiation field pattern of the feed will result in a wider radiation field pattern reflected from the reflector and toward the satellite. This wider radiation field pattern will cause serious interference of the signals from two distinct satellites in the horizontal direction when a wider horn aperture is employed for the dual circular feed horn.